The present invention relates to a method of producing a magnetic disk through magnetic transcription of a magnetic signal.
The magnetic recording/reproducing apparatus has so far been required to have an increased recording capacity. The current tendency of this field is to design a magnetic recording/reproducing apparatus capable of recording data with a higher density. As the typical magnetic storage unit, hard disk drives of more than 1 Gbits/in2 in surface recording density have already been produced on a commercial basis.
Note that the hard disk is a horizontally flat disk having recording tracks formed concentrically in a direction from the central hole toward the outer radius and of which adjacent ones form together a zone in which they are not physically independent of each other but contiguous to each other. In such a hard disk, the radial density of the recording tracks, that is, the track pitch, influences the signal-to-noise ratio (S/N ratio) of the reproduced signal because of the problems which will be explained below.
1. Record Head Width Ww in Disk-radial Direction
When data is recorded by a record head, the larger the width Ww of the record head, the wider the recording mark will be formed. Also, the fringe field of the record head causes fringing in the radial direction of the hard disk so that the recording mark will actually have a width larger than Ww. Therefore, the width of the recording mark includes a part (as wide as the record head) formed by the record head and a part formed due to the fringing. No correct reproduced signal can be obtained from the part formed due to the fringing.
Therefore, when the record head width Ww is as large as the recording track width W, an extra signal will be recorded to adjacent recording tracks and the fringing spread to the recording marks formed on the tracks. If the fringing has thus spread to the recording marks, the S/N ratio of the reproduced signal will be deteriorated when the read head detects the fringe field. Therefore, the record head width Ww should be smaller than the recording track width W (Ww<W).
2. Read Head Width Wr in Disk-radial Direction
On the other hand, to obtain a high-sensitivity reproduced signal from the fringe field of the recording mark, it is necessary to detect no fringe field from the part formed due to the fringing at the time of recording and design the read head width Wr equal to or smaller than the recording mark width. Especially, the read head should be designed to have a width which can cope with the fact that the center of the recording track cannot always be traced because of a servo deviation occurring during data reading. On the other hand, the smaller the read head width Wr, the smaller the obtainable reproduced signal will be. So, the read head width Wr may not be designed smaller because the reproduced signal will have S/N ratio accordingly deteriorated.
Therefore, as will be seen from the description of the problems 1 and 2, the relation among the read head width Wr, record head width Ww and recording track width W will be as follows:Wr<Ww<WSo, a recording mark cannot be formed over the recording track width W while reading cannot be made from a formed recording mark as a whole.
On this account, a technique called “DTR (Discrete Track Recording)” has been proposed for a higher density of data recording to a hard disk. The DTR technique is to solve the aforementioned problems of the recording track pitch. More particularly, in a hard disk, adjacent recording tracks are made independent of each other by forming, between them, a physical recess to such a depth and shape that the fringe field from the recess will not reach the read head or that the read head will not detect the fringe field.
In the hard disk having the recess formed between adjacent recording tracks with the DTR technique, since no consideration may be given to the fringing, so the record head width Ww may be designed larger than the recording track (land) width W and the recording mark may be formed over the land width. Thus, the above problem 1 can be solved. Also, since no consideration may be given to the part formed due to the fringing, the read head width Wr may be designed larger than the land width W. Even with a servo deviation, reading can be made from over the land width W. Thus, the above problem 2 can be solved.
That is to say, the readable recording mark width depends upon the land (recording mark) width W, and the DTR technique is not used. Namely, since the recording mark can be formed wider than the recording mark formed in a hard disk having no recess formed between adjacent recording tracks, so the S/N ratio can be increased in relation to the recording track pitch.
Note here that the hard disk is formed from a magnetic material and information such as servo information is also recorded with magnetic signals of S and N polarities. In the past, a servo signal and disk position signal have been recorded with a jig or the like before the hard disk is assembled into each hard disk drive.
In a hard disk adopting the DTR technique, since the recording tracks are formed as convexities when the hard disk is being produced, so it is necessary to record servo information along the recording tracks. To this end, in Japanese Patent No. 2863190, the Applicant of the present invention proposed a method of recording a servo signal by recording a repetition signal to a hard disk through magnetization with an external alternating magnetic field.
With the improvement in recording density of the hard disk, however, the frequency of the necessary repetition signal is higher, which will cause an external alternating magnetic field. For example, in case the electromagnet is driven with an alternating current, it is difficult to drive the electromagnet proportionally to a higher driving frequency.
Also, in a normal hard disk other than the hard disk produced with the DTR technique, servo information, address information and the like have to be recorded to each recording track, which will take much time.